This invention relates to an improved method for the manufacture by a simple yet highly efficient procedure of iron oxide flakes, particularly .alpha.-iron oxide flakes of a large size otherwise known as micaceous iron oxide (hereinafter referred to as MIO) which manifests an outstanding corrosion-proofing effect when used as a pigment in heavy-duty corrosion-proofing paints.
MIO paints have found widespread acceptance for use in corrosion-proofing iron and steel materials. In Europe, MIO paints have enjoyed popular use since nearly a century ago. As a pigmental component, these MIO paints contain micaceous iron oxide in high concentrations. MIO means naturally occurring micaceous, scaly or flakelike crystals of .alpha.-Fe.sub.2 O.sub.3. These natural MIO's contain about 15% by weight of silica sand, alumina and other impurities. They occur Austria, Germany, the United Kingdom, etc. Among these, particularly excellent both qualitatively and quantitatively are the MIO produced in Austria. In coats formed of MIO paints, the thin flakes of .alpha.-Fe.sub.2 O.sub.3 are arranged in a stratal form throughout the coats of paints so as to intercept ultraviolet rays and obstruct moisture and gases. When they are used as overcoats or intermediate coats, therefore, they protect their respective undercoats and extend the service life of the substrates. This means that the thin flakes of iron oxide which are arranged in a stratal form in the coats are particularly effective in protecting the substrates from corrosion. MIO paints have found widespread utility in applications to virtually all sorts of steel materials which are used in bridges, electric poles, tanks and other structures including even the famous Eiffel Tower in Paris. In Japan, MIO paints began to appear on the market about a decade ago. They have so far been used for coating giant bridges such as the Kammon Bridge and the Great Kobe Bridge. MIO paints have been designated for use on bridges to be constructed across the Seto Inland Sea. Since the estimated deposits of natural MIO of high quality in Europe are not very large, it is uncertain whether a stable supply of MIO will be available in future years.
In view of this uncertainty, the inventors continued a study in search for a method for synthetic production of MIO and achieved good results. Some of the inventions which have issued from their study are enumerated below.
Examples of inventions relating to methods for manufacture are:
Japanese Patent Publication (hereinafter abbreviated as J.P.P.) SHO No. 43(1968)-12435, claiming: "A method for the manufacture of .alpha.-iron oxide, which comprises dispersing an .alpha.-iron (III) hydroxide oxide or basic iron (III) salt in an aqueous alkali solution and subjecting the resultant dispersion to a hydrothermal treatment at temperatures of not less than 250.degree. C.".
J.P.P. SHO No. 45(1970)-54, claiming: "A method for the manufacture of MIO, which comprises compression molding iron (III) hydroxide oxide or basic iron (III) salt into highly packed tablets, placing the tablets in the aqueous solution of an alkali and an alkaline earth metal hydroxide, and subjecting the resultant mixture to a hydrothermal treatment."
J.P.P. SHO No. 48(1973)-29718 (corresponding to U.S. Pat. No. 3,987,156, B.P. No. 1,333,789 and French Patent No. 7,040,844), claiming: "A method for the manufacture of MIO and sodium sulfate, which comprises producing a hydrate paste of iron (III) and an alkali from a thick aqueous solution of iron (III) and a hydrated strong alkali solution, and subjecting this hydrate paste to a hydrothermal treatment in an excess alkali solution."
J.P.P. SHO No. 49(1974)-44878, claiming: "A method for the manufacture of MIO by the steps of dispersion iron (III) hydroxide in an aqueous alkali solution and subjecting the resultant dispersion to a hydrothermal treatment, which method is characterized by using iron (III) hydroxide which is produced by adding an aqueous iron (III) salt solution to an aqueous alkali solution." Examples of inventions relating to applied techniques are:
J.P.P. SHO No. 50(1975)-24156, claiming: "A method for the manufacture of a ferromagnetic oxide material, which comprises subjecting iron (III) hydroxide oxide, basic iron (III) salt, iron (III) hydroxide and amorphous iron (III) hydroxide to a hydrothermal treatment in an aqueous alkali solution to produce MIO, mixing this MIO with other metal salt or metal hydroxide and burning the resultant mixture."
J.P.P. SHO No. 51(1976)-10261, claiming: "A resinous composition for the manufacture of reinforced molded articles of resin, comprising a synthetic resin and MIO contained in a dispersed state in the synthetic resin."
The method the inventors previously developed for the manufacture of the synthetic MIO comprises the steps of preparing iron hydroxide, for example, from an iron salt and subjecting the iron hydroxide to a hydrothermal treatment in an aqueous alkali solution for producing crystalline iron oxide flakes. In this case, the spent pickling solution issuing from the secondary iron and steel industry, the by-produced iron sulfate issuing from the titanium oxide industry or some other spent iron salt can effectively be utilized as the iron salt. Thus, the above method, aside from producing an useful substance, serves the dual purpose of preventing environmental pollution by industrial effluents and reclaiming otherwise wasted resources. A typical process in which the above method is practiced by using the by-produced iron sulfate from the titanium oxide industry is illustrated in the accompanying drawing.
In this process, a divalent iron sulfate is oxidized into a trivalent iron sulfate and this trivalent iron sulfate is neutralized with caustic soda to induce an amorphous precipitate formed preponderantly of iron hydroxide. Then, the amorphous precipitate is placed in an autoclave and subjected therein to a hydrothermal treatment. This hydrothermal treatment produces crystalline MIO and a mother liquid which is a mixed aqueous solution containing caustic soda and sodium sulfate. The crystalline MIO is separated from the mother liquid (aqueous solution) through filtration. The crystals thus separated are washed with water, dried and thereafter perpared as the final product. The mother liquid is sent to a Glauber's salt plant, where it is cooled to give rise to Glauber's salt crystals. The caustic soda solution which consequently occurs is put to reuse.
The synthetic MIO produced as described above is compared with natural MIO with respect to main properties in Table 1.
TABLE 1 ______________________________________ Synthetic MIO and natural MIO Item Synthetic MIO Natural MIO ______________________________________ Purity Not less than 98% 80 to 85% Shape of particles Thin hexagonal Preponderantly plates flaky particles Particle size Fairly uniform Not uniform distribution Size ratio* About 1:10- 30 Too ununiform to permit measurement Particle diameter** 2 to 60 .mu. Too irregular to permit measurement Adjustment of Feasible (2 to 60 .mu.) Not feasible (crush- particle size ing results in loss of shape) Water-soluble 0.005 to 0.015% 0.340 to 0.473% content Absorption Wavelengths of not -- spectrum more than 560 m.mu. absorbed Thermal resistance Crystals retained Crystals disinteg- intact up 1100.degree. C. rate at 800.degree. C. Resistance to acids Not affected by 1.0N -- and alkalis sulfuric acid Resistance to Not affected by -- acids and alkalis strong alkalis ______________________________________ *Ratio of thickness to length of plate **Length of plate
Paints comprising synthetic MIO are superior to paints comprising natural MIO in many points.
It is clear from Table 1 that synthetic MIO is decidedly superior to natural MIO in appearance and physical constants. Since the surface of the minute flakes of the synthetic MIO is very smooth, the crystalline surface reflects incident light and shines brilliantly and, therefore, assumes a beautiful appearance. The sparkling effect of the flakes is particularly pleasant in the case of giant MIO crystals more than 30 microns in size. In contrast, the particles of the natural MIO contain more impurities than the synthetic MIO, possess no definite shape and vary greatly in particle size. Thus, the natural MIO gleams dully. The flake surface of the thin single crystal of the synthetic MIO corresponds to the surface perpendicular (0001) to the axis C of the hexagonal system. In the absorption spectrum of the crystal of the synthetic MIO, all the short wavelength rays of not more than 560 m.mu. are absorbed. This fact explains why the synthetic MIO crystals arranged in a stratal form in a coat of MIO paint provides perfect interception to harmful ultraviolet rays. Thus, the MIO paints enjoy high weatherability over a long period of time. Besides, the synthetic MIO exhibits outstanding thermal stability and offers high resistance to acids and alkalis and is suitable for use as a pigment in the range of from pH 8.0 to pH 8.7.
Thus, there has been developed a synthetic MIO which far excels the natural countertype in numerous respects. The effect of the MIO paint manifested in its corrosion-proofing activity increases in proportion as the flake surface of the thin single crystals of MIO increases. A need has been felt, accordingly, for the development of a method capable of producing large MIO particles. Methods so far developed for the manufacture of such large MIO particles may be roughly grouped as follows:
(1) Methods (such as is disclosed by J.P.P SHO No. 45(1970)-54) which comprise the steps of compression molding .alpha.-iron (III) hydroxide oxide and subjecting the molded particles to a hydrothermal treatment.
(2) Methods (such as is disclosed by J.P.P. SHO No. 48(1973)-29718) which involve increasing the alkali concentration of the mother liquid for the hydrothermal treatment.
(3) Methods (such as is disclosed by J.P.P. SHO No. 49(1974)-44878) which adopt a reverse neutralization technique for the preparation of iron hydroxide precipitate.
(4) Methods which involve using suitable amounts of seed crystals in the hydrothermal treatment of iron hydroxide.
By a suitable combination of methods described above, large MIO particles up to about 60 microns of particle diameter can be prepared in a laboratory,. No technique has yet been developed for commercial production of large MIO particles exceeding 30 microns in particle diameter.
An object of the present invention is to provide a method for the manufacture of large MIO particles by a commercial operation.